Co-pending patent application 09/248,009 now U.S. Pat. No. 6,241,851 filed on Feb. 10, 1999 (the complete disclosure of which is included by reference herein), discloses a method of treating comminuted cellulosic fibrous material with a beneficial additive prior to chemical digestion. This additive is preferably a strength- or yield-enhancing additive, such as athraquinone [AQ] or polysulfide [PS] and their derivatives or equivalents. The present invention comprises a further method and apparatus for effecting the pretreatment of comminuted cellulosic fibrous material, typically wood chips (though the invention is equally applicable to the treatment of other forms of cellulose), to improve the properties [e.g. strength] of the resulting pulp or to improve the effectiveness [e.g. yield] of the pulping process.
It has been discovered that the pretreatment of comminuted cellulosic fibrous material, for example, wood chips, can be more effective if, among other things, the heating and cooking of the chips after pretreatment is essentially isolated from the pretreatment process. That is, a more effective pretreatment can be obtained if the pretreatment process is performed at a cooler temperature, with or, preferably substantially without, the presence of alkali, and the heating of the chips to cooking temperature is performed after the pretreatment (impregnation of the cellulose material) is essentially completed. In one aspect of this invention the additive is added earlier in the treatment process, and the content of cooking chemical, for example, kraft white liquor, is reduced or diverted from this earlier stage of pretreatment and introduced during later stages of pretreatment or in the formal cooking treatment. Thus, according to the present invention, possibly longer, cooler, less alkaline pretreatment is provided so that the cooking additive more effectively treats the chips prior to heating to cooking temperature, that is, to a temperature greater than 140xc2x0 C.
According to one aspect of the present invention there is provided a method of continuously producing chemical cellulose pulp from a comminuted cellulosic fibrous material slurry, comprising substantially continuously: (a) Impregnating the material with a solution containing yield or strength-enhancing additive at effective alkali and temperature conditions so that substantially no alkali degradation of the cellulose occurs, and so that little or substantially no acid hydrolysis occurs. And, (b) after (a), treating the material with an alkaline cooking liquor, at cooking temperature, to produce a chemical cellulose pulp with higher yield or strength than if (a) were not practiced.
In the method (a) may be practiced so that there also is substantially no dissolution of lignin from the material. At the end of (a), the material has been impregnated with the yield or strength enhancing additive (such as AQ or its derivatives or equivalents) so that the problems associated with conventional higher temperature additive impregnation are avoided. For example specifically with respect to AQ, it is a large molecule and needs a longer time to diffuse into the wood chips, or like cellulose material, than does, for example, alkali, and it needs to be reduced in order to dissolve so that it can diffuse. Therefore sometimes AQ is used in its reduced form (commonly referred to as SAQ). However also typically about 80% of the AQ reacts with dissolved lignin and thus is not capable of performing its intended yield and strength enhancing function, leaving only about 20% of the AQ for performing the desired functions. By utilizing the invention a much higher percentage of the AQ (up to substantially all) that is added actually impregnates the wood chips and performs its yield or strength enhancing function, and the AQ may or may not be added in reduced form. When the AQ successfully impregnates the chips it keeps the hemicellulose from being dissolved during cooking, and thus increases yield, and utilizes other mechanisms to enhance strength. Other materials may also be utilized to facilitate penetration of the chips with the additive, such as the use of a surfactant.
In the method as described above (a) may be practiced at an alkali concentration between 0-less than 10 g/l expressed as NaOH, and at a temperature of between about 80-130xc2x0 C., typically less than about 120xc2x0 C., and preferably about 110xc2x0 C. or less. The amount of alkali is most desirably substantially zero, but preferably at most less than about 5 g/l expressed as NaOH.
In the method (a) may be practiced in a feed system of a continuous digester and (b) in a continuous digester. Exactly where in the feed system (a) is practiced may be widely variable. For example a chip bin could be located in a wood yard, and the additive could be sprayed or otherwise applied to the wood chips even prior to entry into the chip bin, or while they were entering the chip bin, or while they were in the chip bin. Then the wood chips could be pumped from the wood yard to the digester using primarily or substantially exclusively water as the slurrying medium (with no intentional significant alkali addition) so that the chips would be at desirable low temperature, low alkali, impregnation-facilitating conditions for a significant period of time. In a typical situation impregnation in (a) will take at least about 20 seconds, e.g. between about 2-60 minutes at superatmospheric pressure (which superatmospheric pressure may be provided in any conventional manner including by a level of liquid above the chips, pumping, and/or in a pressure vessel). Alternatively the additive may be introduced after the chip bin and before a pump and/or high pressure feeder, in a separate treatment vessel such as an impregnation vessel, or any other location in the feed system that proves advantageous for any reason.
The invention may also comprise substantially immediately after impregnation in (a) adding between about 35-100% of the alkali used to treat the material, including in (b), to the material. Alternatively the alkali can be added far downstream. Also the method may further comprise (c), between (a) and (b), treating the material in a second zone with a solution containing additive at an effective alkali concentration of about 5-less than 15 g/l expressed as NaOH and greater than in (a), and at a temperature of between about 110-130xc2x0 C. and higher than in (a)
In the method (a) may be practiced using as the additive at least one of AQ or its derivatives or equivalents, polysulfide or its derivatives or equivalents, or sulfite in the form of sulfur dioxide, NaHSO3, or Na2SO3. A suitable amount of additive may be used; for example if AQ or its derivatives or equivalents are used, typically (a) is practiced with a total additive concentration of between about 0.02-0.5% on wood, typically between about 0.02-0.1% on wood.
Additive may also be added during the digesting process, as is conventional.
According to another aspect of the present invention there may be provided a method of treating comminuted cellulosic fibrous material comprising substantially continuously: (a) Treating a slurry of comminuted cellulosic fibrous material with a first liquid comprising primarily or substantially exclusively water containing a beneficial additive at a maximum effective alkali concentration of less than 10 g/l, and at a first temperature of about 80-130 degrees C., for at least about 20 seconds, e.g. between about 2-60 minutes under superatmospheric pressure. And, (b) after (a), treating the material in the slurry with a second liquid having an initial effective alkali concentration greater than 10 g/l (preferably greater than 15 g/l and most desirably greater than 20 g/l) expressed as NaOH, and at a second temperature greater than 130xc2x0 C. (e.g. greater than 140xc2x0 C.), to produce a chemical cellulose pulp.
The method as described above may further comprise (c), between (a) and (b), treating the slurry with a third liquid containing the beneficial additive at a maximum effective alkali concentration greater than the first liquid and less than the second liquid, and at a third temperature, higher than the first temperature and less than 140 degrees C.
Another embodiment of the present invention may be a method of treating comminuted cellulosic fibrous material consisting of or comprising: (a) treating (e.g. pretreating) a slurry of comminuted cellulosic fibrous material with a liquid containing a beneficial additive at a first alkali content (e.g. from 0-less than 10 g/l) at a first temperature; (b) after (a), treating the slurry with a second liquid containing a beneficial additive at a second effective alkali content [greater than zero, and preferably at least 10% greater than the first effective alkali content] at a second temperature higher than the first temperature; and (c) after (b), treating [possibly, although not necessarily, after displacement of substantially all of the additive therefrom] the slurry with a third liquid having an effective alkali content (i.e. concentration) greater than the first and second effective alkali contents and a temperature greater than the first and second temperatures and greater than 130xc2x0 C. (e.g. greater than 140xc2x0 C.) to produce a chemical cellulose pulp.
As described above, the beneficial additive used in steps (a) and (b) may be AQ (most desirably SAQ), polysulfide, sulfur, surfactants, and combinations thereof. The first alkali content, expressed as xe2x80x9ceffective alkalixe2x80x9d (EA) as NaOH, is preferably less than 10 g/l, and may be less than about 5 g/l, or the first liquid may contain no alkali at all. For example, the first liquid may be mill water, steam condensate, or washer filtrate containing little or no alkali content. The first temperature of treatment (a) is preferably a relatively cool temperature, that is, a temperature less than 130xc2x0 C., preferably less than about 120xc2x0 C., for example, the first temperature may range from about 90 to 110xc2x0 C., or be even lower.
Procedure (a) may be performed using the methods and apparatuses described in U.S. Pat. Nos. 5,476,572; 5,622,598; and 5,632,025, that is the system marketed under the trademark LO-LEVEL(copyright) feed system by Andritz-Ahlstrom Inc. of Glens Falls, N.Y. The LO-LEVEL(copyright) feed system, that is a system employing a chip pump and not using a horizontal xe2x80x9csteaming vesselxe2x80x9d, is particularly suited for treatment according to the present invention since this system allows for the feeding and treatment of chips at lower temperatures than can be handled by conventional feed systems. Other conventional equipment and processes may also be used to perform the present invention, for example, those not including a chip pump and including a horizontal steaming vessel.
The second liquid of step (b) preferably contains at least some alkali, for example, the second liquid contains less than 15 g/l EA, typically about 5 to 10 g/l EA. This alkali content may be supplied by kraft white, green, or black liquor, or from combinations of white, green, or black liquor and water or washer filtrate. The second temperature is also preferably less than 140xc2x0 C., for example, less than about 120xc2x0 C., and is typically between about 110 and 130xc2x0 C. The procedure (b) is typically performed in the upper part of continuous digester, for example, a Kamyr(copyright) continuous digester as also sold by Andritz-Ahistrom Inc. of Glens Falls, N.Y.
Procedure (c), the formal pulping process, may be any form of chemical pulping process, but is preferably one or more of the processes described in U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; 5,662,775 and others. The processes and apparatuses described in these patents are marketed under the name LO-SOLIDS(copyright) Pulping by Andritz-Ahlstrom.
According to another aspect of the present invention there is provided a method of continuously producing chemical cellulose pulp from a comminuted cellulosic fibrous material slurry, comprising continuously: (a) In a first treatment zone treating the material with a solution containing yield or strength-enhancing additive at effective alkali and temperature conditions so that substantially no alkali degradation of the cellulose occurs (and so that the material and additive flow in contact with each other for a period of time). (b) After (a), treating the material in a second zone with a solution containing additive at an effective alkali concentration of about 5-less than 15 g/l expressed as NaOH, and at a temperature of between about 110-130xc2x0 C. and higher than in (a). And (c) after (b) (and possibly, but not necessarily desirably, after removing substantially all of the additive from contact with the material), treating the material with an alkaline cooking liquor, at cooking temperature, to produce a chemical cellulose pulp with higher (e.g. at least 2% higher) yield or strength than if (a) and (b) were not practiced.
In the method preferably (a) is practiced in a feed system of a continuous digester, (b) is practiced in a top zone of the continuous digester, and (c) is practiced in the continuous digester below the top zone. Also, preferably (a) is practiced at an alkali concentration between 0-less than 10 g/l expressed as NaOH, and less than in (b), and at a temperature of between about 80-110xc2x0 C. At least 50% of the additive may be introduced in (a), or about 40% may be introduced in (a) and 60% of the additive introduced elsewhere, for primary use in (b). In an embodiment wherein the additive is AQ or its derivative or equivalents, and wherein (a) and (b) are practiced with a total additive concentration of between about 0.05-0.15% on pulp.
The apparatus used in practice of the present invention is primarily conventional apparatus, except, for some aspects, for the additive additions to the feed system, and to the top zone of a continuous digester, and the potential recirculation of additive withdrawn from the top zone (including a screen at a transition between the top zone and the cooking zone of the continuous digester) and introduction to the feed system.
According to another aspect of the present invention there is provided a cellulose slurry treatment system comprising: An upright continuous digester having an inlet adjacent the top and an outlet adjacent the bottom thereof. A feed system for the digester including a slurry pump. A yield or strength-enhancing additive conduit for introducing yield or strength-enhancing additive into the slurry before the slurry pump. A top treatment zone of the digester, and a screen assembly adjacent the bottom of the top treatment zone, and a cooking zone below the screen assembly. The screen assembly providing a temperature transition within the digester. Means for introducing or re-circulating liquids into the digester so as to establish upward flow of liquid in the digester above the screen. And means for introducing yield or strength-enhancing additive into the top zone and/or feed system of the digester.
In the system, the means for introducing additive into the feed system may comprise conventional conduits, nozzles, venturis, or other conventional structures capable of introducing a liquid into a flowing liquid or slurry. The means for introducing liquid into the digester to ensure upward flow immediately adjacent the screen assembly (which preferably comprises the second screen in the digester going from the top to the bottom) comprises any conventional structure that will accomplish that purpose, including re-circulation conduits with central pipes connected to pumps, etc. The invention also preferably comprises means for re-circulating additive withdrawn from the top zone of the digester to the additive introduction means associated with the feed system. Such re-circulation means may comprise conventional conduits, and/or pumps, valves, or like fluid structures for that purpose, and may also include separation equipment for separating the additive from some of the liquid with which it is re-circulated. The preferred additives are those described in co-pending application Ser. No. 09/248,009.
It is the primary object of the present invention to provide for the effective treatment of comminuted cellulosic fibrous material so as to increase the strength, yield, and/or other advantageous properties of the pulp or the treatment process. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.